i^e, 0 -UJ £ , TRANSACTIONS OF THE SAN DIEGO SOCIETY OF NATURAL HISTORY VOLUME XII. No. 10, PP. 181-205, 1 MAP, PLATE 13 UPPER PLEISTOCENE MOLLUSCA FROM POTRERO CANYON, PACIFIC PALISADES, CALIFORNIA BY JAMES W. VALENTINE Department of Geology University of California, Los Angeles SAN DIEGO, CALIFORNIA PRINTED FOR THE SOCIETY JULY 2, 1956 LIBRARY LOS t\>\thlS.S COUNTY MUSEUM, hXiK v^TJOM ,?K COMMITTEE ON PUBLICATION LAURENCE M. KLAUBER, Chairman JOSHUA L. BAILY CHARLES C. HAINES CARL L. HUBBS JOHN A. COMSTOCK, Editor TABLE OF CONTENTS Index map of Pacific Palisades 184 Introduction Previous work Present work 187 Acknowledgments 187 Stratigraphy of the terrace deposits 188 Paleoecology 189 Habitat 189 Temperature 190 Inferred depositional environment 192 Age and faunal affinities 192 Checklist of fossils 194 Key 194 Systematic checklist 194 References 202 Plate 13 205 FIGURE 1. Index map of a portion of Pacific Palisades, California, showing the locality from which the Clark collection was recovered (UCLA Loc. no. 3225) and its relation to the shoreline during maximum sea stand on the Dume terrace platform. UPPER PLEISTOCENE MOLLUSCA FROM POTRERO CANYON, PACIFIC PALISADES, CALIFORNIA BY JAMES W. VALENTINE INTRODUCTION Fossiliferous marine terrace sands of Upper Pleistocene age are exposed near the head of Potrero Canyon, Pacific Palisades, California (figure 1). Tens of thousands of mollusks from this deposit, collected over a period of years by Dr. F. C. Clark, were acquired by the Depart- ment of Geology, University of California, Los Angeles, in January, 1943. The preparation of homesites and other construction in Potrero Canyon has caused removal or burial of the most fossiliferous portions of the terrace sands, so that Clark's original locality is destroyed, and his collection cannot now be duplicated. It is therefore desirable to record this large assemblage, which contains 128 species and varieties previously unreported from the Upper Pleistocene of Potrero Canyon. PREVIOUS WORK Ralph Arnold was apparently the first to assign strata at Potrero Canyon to the Upper Pleistocene; he referred to the soft, rudely stratified nonmarine deposits forming much of the Palisades at Potrero Canyon and southward as probable equivalents of the Upper San Pedro series, "from lithological and stratigraphical reasons" (1903, p. 56). This correlation cannot be validated at present, although it is likely that some of the nonmarine terrace cover is of Upper Pleistocene age. Marine deposits that are now considered Upper Pleistocene were first reported from the Santa Monica Mountains in 1904 by Rivers, who mentions "Quaternary beach gravels" as capping strata that he called Pliocene. Rivers described two mollusks, Cdvolin<a telemus var. tricuspida and Strombiformis raymondi, from the Upper Pleistocene. It is probable that these came from Potrero Canyon. 186 SAN DIEGO SOCIETY OF NATURAL HISTORY Following Rivers' paper, several records of species from Upper Pleistocene deposits at "Santa Monica" appear in the literature. These forms are with but one exception (Anachis lineolata) present in the Clark collection, and are presumed to be from Potrero Canyon. The earliest of these papers is by Raymond (1906), who recorded varieties of Megasurcula carpenteriana from the "Pleistocene of Santa Monica", attributing the collection to Rivers. Bartsch described Alabina monicensis from the "Upper San Pedro series at Santa Monica, California" in 1911, and in 1917 described a new species of Balcis (monicensis) and recorded three additional species of this genus (micans, oldroydi, and rutila) from the same locality. The first chitons were reported from "Pleistocene deposits of the Santa Monica Hills" by Chace (1917), who recorded 7 identified forms collected by Clark. More gastropods were described from the "Upper Pleistocene of Santa Monica" by T. S. Oldroyd in 1921 (Anachis minuta = A. lineolatd, and Epitonium clarki), and a record of Acteocina infrequens by Dall (1922), from an unspecified locality in Santa Monica, may belong here. The earliest specific reference to an Upper Pleistocene fossil locality in Potrero Canyon appears to have been made by Berry (1922, p. 412, as "Long Wharf Canyon"). Working with material supplied by Clark, he revised and expanded Chace's list of chitons to 9 species. It remained however for Hoots to locate accurately the Potrero Canyon exposures on a map (1931, pi. 16), accompanied by the most complete description of the fauna yet published (Woodring in Hoots, 1931, pp. 121-122). Woodring listed 121 forms, of which 89 were specifically identified. The fauna was interpreted as a warm-water assemblage, cor- relative with the Upper San Pedro of Deadman's Island (Palos Verdes sand). Differences between the San Pedro and Potrero Canyon Upper Pleistocene assemblages were ascribed to biofacies differences. A considerable number of species from the Upper Pleistocene of "Santa Monica", presumably Potrero Canyon, were included among the Pleistocene records compiled in Grant and Gale's famous catalogue (l93l). These records are based on specimens in the Oldroyd and Clark collections (now at Stanford University and the University of California, Los Angeles, respectively). More recently, Woodring has re-emphasized the southern aspect of the Potrero Canyon fauna while affirming his earlier correlations (in Woodring, Bramlette, and Kew, 1946, p. 106). POTRERO CANYON PLEISTOCENE. J. W. VALENTINE 187 PRESENT WORK Preparation and identification of the F. C. Clark collection at U.C.L.A. was begun by Mr. E. H. Quayle in 1944, but was put aside due to the press of other duties. This work was completed by the writer in 1955 during the course of studies of Pleistocene molluscan faunas. The material received from Dr. Clark included Pleistocene mollusks from localities other than Potrero Canyon, which had un- fortunately become mixed with a few of the Potrero Canyon fossils. Material so contaminated had been preserved separately by the direction of Professor W. P. Popenoe. Attempts to identify the source of this mixed material proved fruitless, as much of it had been cleaned of matrix. It has accordingly been excluded from the present study. There remain in the Clark collection 227 species and varieties of mollusks that are unquestionably from the Upper Pleistocene deposits in Potrero Canyon (U.C.L.A. Locality no. 3225). The fauna is herein listed, together with all known published records of fossils from the same locality. In listing previous records, the nomenclature has been revised to accord with present usage. The chitons were identified by Dr. S. Stillman Berry during the preparation of his monograph (Berry, 1922). A total of 262 molluscan species and varieties are now reported from Potrero Canyon Upper Pleistocene. Species or varieties not previously reported as fossils are Acteocina magdalenensis, Sulcoretusa xystrum (plate 13, fig. 10), Hanetia elegans (plate 13, figs. 7, 8), Odostomia farallonensis, Odostomia gravida, Ala- bind tenuisculpta var. phalacra, and Polinices draconis. ACKNOWLEDGMENTS Professors U. S. Grant and W. P. Popenoe, University of Cali- fornia, Los Angeles, and Dr. John T. McGill, Geologist, U. S. Geological Survey, aided the preparation of this paper by careful criticism and discussion. The Clark collection was made available to the writer through the courtesy of Mr. Takeo Susuki, Junior Research Geologist, University of California, Los Angeles. Dr. Leo G. Hertlein, California Academy of Sciences, kindly furnished comparative material and ecologic data on certain mollusks. The index map was drafted by Mr. Ronald H. Arntson, University of California, Los Angeles, and Mr. Armour C. Winslow, Louisiana State University, aided in the preparation of the plate. 188 SAN DIEGO SOCIETY OF NATURAL HISTORY STRATIGRAPHY OF THE TERRACE DEPOSITS Remnants of a marine platform of abrasion, included by Davis in his Dume terrace (1931, p. 1098 ff.), are preserved beneath an alluvial cover in the Pacific Palisades region. The platform surface is eroded on tilted strata of considerable structural complexity, which range in age from Paleocene to Lower Pleistocene. Near the head of Potrero Canyon, about 0.1 mile south of U.C.L.A. Locality no. 3225 (figure l), this surface is exposed in stream and road cuts. It is underlain here by gently tilted, gray-green, massive siltstones mapped as Pliocene by Hoots (1931, pi. 16), and overlain by 1 to 6 feet of essentially horizontal, iron-stained, crudely bedded, loosely consolidated, fine to medium grained, angular quartz sands. The sands, where exposed, are largely unfossiliferous; only a few scattered sponge spicules and small, badly worn shell fragments could be found. About 50 feet of alluvium overlie the sands. The surface of the terrace platform is somewhat irregular, as is well shown in Davis' photographs (1933, pi. 55). In Potrero Canyon, at the locality mentioned above, a shallow depression in the platform surface is exposed on the east bank of the stream. Very fine, highly fossiliferous sand fills this depression; it is finer than the sands above but coarser than the underlying siltstones. The molluscan fauna is similar to that in the Clark collection; in addition, foraminifera, ostra- codes, and the minute skeletal remains of various other groups of marine invertebrates are present. Hoots reports that the locality from which the Clark collection was recovered (locality 61 of Hoots, 1931, p. 121) is in a fine white and brown sand resting directly upon the Pliocene siltstones. The matrix of the mollusks in the Clark collection agrees well texturally with the very fine fossiliferous sand exposed in the depression, and contains a similar microfauna. It seems likely therefore that the rich Upper Pleistocene fauna has come from a unit lying between the Dume plat- form surface and the barren, fine to medium grained sand exposed in Potrero Canyon today. The fossiliferous unit was apparently stripped off much of the terrace before the overlying sands were deposited. Among the most abundant foraminifera from the fossiliferous unit are Bolivina interjuncta Cushman, Uvigerina peregrina Cushman, s. L, Cassidulina californica Cushman and Hughes, Cassidulim trans- lucens Cushman and Hughes, and Anomalina cf.